1. Field of the Invention
The present invention relates to a nonvolatile semiconductor memory device and a method for manufacturing or fabricating the nonvolatile semiconductor memory devices.
2. Description of the Related Art
In recent years, the EEPROM (Electrically Erasable and Programmable Read Only Memory) is finding rapidly increasing use as the fields of application, such as mobile phones and digital cameras, keep expanding. An EEPROM that has a capability of entire or block erasure of data electrically is called a flash EEPROM.
An EEPROM is a nonvolatile semiconductor memory device that stores binary or multi-level (multi-valued) digital information depending on whether a predetermined amount of electric charge is accumulated in a floating gate or not and reads out the digital information according to a change in current flow through a channel region relative to the amount of electric charge.
With the conventional EEPROM, it is necessary to apply a voltage on a control gate, which is stacked on the floating gate, in order to transfer electric charge to and from the floating gate. Hence, there must be provided a wiring for the control gate for each memory cell, which results in a complex structure of a memory cell. Also, with the conventional EEPROM, it is necessary to apply a high voltage to a source or a drain in order to inject electrical charge into or eject it from the floating gate.
In the manufacture of the conventional EEPROM, the process for fabricating a control gate is a requisite, presenting a problem of incompatibility with a logic process. Especially when an EEPROM is used for small-capacity data storage of several bytes or when there is a mixed presence of logic circuitry and EEPROM, the necessity for a fabrication process of the EEPROM independently of the logic process raises manufacturing costs.